Contact structures and methods for making same

ABSTRACT

Contact structures are formed by building a core structure on a substrate and over coating the core structure with a material that is harder or has a greater yield strength than the material of the core structure. The core structure may be formed by attaching a wire to the substrate and spooling the wire out from a spool. While spooling the wire out, the spool may be moved to impart a desired shape to the wire. The wire is severed from the spool and over coated. As an alternative, the wire is not over coated. The substrate may be an electronic device, such as a semiconductor die.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/290,639, filed Nov. 7, 2002 now U.S. Pat. No. 6,818,840, which is acontinuation of U.S. patent application Ser. No. 09/245,499, filed Feb.5, 1999 (abandoned), which is continuation of U.S. patent applicationSer. No. 08/457,479, filed Jun. 1, 1995 (now U.S. Pat. No. 6,049,976),which is a divisional of U.S. patent application Ser. No. 08/152,812,filed Nov. 16, 1993 (now U.S. Pat. No. 5,476,211).

BACKGROUND OF THE INVENTION

This invention relates in general to electronic assemblies and testingthereof. In particular, the invention relates to a method of manufactureof protruding, controlled aspect ratio and shape contacts for uses ininterconnections of assemblies and testing thereof.

Interconnections which involve protruding electrical contacts are usedextensively in packaging of electronics. Pin grid array packages, bothplastic and ceramic, housing a variety of semiconductors, use areaarrays of pins as interconnect contacts for connection to circuitboards. Pins can be attached to their receiving package conductors byuse of a variety of methods. For ceramic packages, pins are insertedinto non-reacting brazing fixtures and are then gang-brazed tocorresponding conductive terminals on the package. This approach ischaracterized by significant non-recurring engineering costs and leadtimes involved in production of the brazing fixture. Plastic pin gridarray packages most commonly use pins which are inserted into metallizedthrough holes in a circuit board, while the dimensions of pins and theholes normally chosen to facilitate good contact between the walls ofthe pins and the coating of the holes. This approach has a disadvantagein that the coated holes and the pins block some circuit routingchannels within the circuit board, thus forcing either use of narrowcircuit traces, or increase in circuit board area, either of whichresults in increased costs.

Permanent connection of the pin grid array packages to circuit boardsoften is accomplished by inserting pins through corresponding holes in acircuit board, the pins protruding to a predetermined length beyond thecircuit board. A resulting assembly then is passed through a wavesoldering machine, and the pin grid array thus is soldered to thecircuit board. Alternatively, a pin grid array can be inserted into alow insertion force or zero insertion force socket for a demountableassembly. Such a socket, in its turn, normally is connected permanentlyto a board.

A current trend in interconnections is toward face-to-face surfacemounting of components to boards and semiconductor chips to substrates.This approach is best accomplished with protruding contact structures ontop of (or otherwise protruding from) contact carrying conductiveterminals or traces. Conductive terminal arrangements on facingcomponents and substrates are increasingly being made of the area arraytype, as this allows for larger contact-to-contact separation ascompared with components characterized by peripheral arrangement ofinterconnection contacts.

Pins attached to either ceramic or plastic packages according to thetraditional methods are, in general, not appropriate for mounting topatterns of surface contacts on circuit boards, due to pin lengthvariation. For surface mounting, the pins would have to be planarized,which represents an additional expensive step subsequent to pinassembly. In addition, there is a significant cost penalty associatedwith production of pin-carrying packages with pin-to-pin separations of50 mils, or lower.

There is currently an increasing need for a low cost method of attachingprotruding contacts from conductive terminals, arising fromproliferation of surface mountable area array contact packages.Stand-off height of protruding contacts is particularly important whencoefficients of thermal expansion of components and of circuit boardmaterials differ significantly. The same is true for attachment ofun-packaged semiconductor chips to interconnection substrates. Theseexpansive concerns call for a low cost, high volume method ofmanufacturing protruding, controlled aspect ratio or shape electricalcontacts on top of (or otherwise protruding from) contact carryingconductive terminals, on top of any device or circuit bearing substrate,board material or component, and its applications to surface mountinterconnections of devices, components and substrates.

THE PRIOR ART

U.S. Pat. Nos. 5,189,507, 5,095,187 and 4,955,523 disclose a method ofmanufacturing of controlled height protruding contacts in a shape ofwires for direct soldering to a mating substrate. The wires-are bondedto terminals without use of any material other than that of wire and theterminals, using ultrasonic wirebonding methods and equipment, whichcomprises a standard industry technique for interconnectingsemiconductor chips to packages. The patents also describe a bondinghead which incorporates a wire weakening means for controlling length offree standing severed wires. Vertically free standing wires present ahandling problem during assembly, which is addressed in the patents byproviding for polymer encapsulation of bonds between the wires andterminals. The polymer coating, which is optional, also compensates foranother disadvantage of the approach, namely weak points along the wire,typically the point of contact between the wire and terminalmetallization, and in case of ball bonding, in a heat effected zone ofthe wire just above impression of a bonding capillary. While thesepatents provide for controlled height contacts, and discuss 2d to 20 daspect ratios, in practice they do not assume controlled aspect ratiosfor all kinds of protruding contacts which are required in variousapplications. For instance, standard, high speed wirebonding equipmentcould not handle a 30 mil diameter wire. Therefore, according to theseinventions, a 30 mil diameter, 100 mil high contact could only beproduced on lower throughput specialized equipment, at higher cost. Inaddition, a gold wire as described in a preferred embodiment, would havea problem of dissolving in solder during a soldering cycle, which causeslong term reliability problems with solder joints similarly, directsoldering of copper contacts would in many cases result in undesirablereaction between copper and solder at elevated temperatures. Whilenickel metal-is the material of choice for solder joint reliability,nickel wire can not be used for ultrasonic wirebonding to metalterminals due to its high mechanical strength and passivating, oxideforming properties. Chemical, physical and mechanical properties, aswell as permissible dimensions and shapes of the protuberant contactsproduced according to this invention are limited to the capabilities andmaterials choices compatible with known wire bonding techniques.

U.S. Pat. No. 3,373,481 describes a method of interconnectingsemiconductor components on substrates by means of dissolving protrudinggold projections on the components in solder masses formed on thesubstrate terminals. The gold projections are formed by compression andextrusion of gold balls against the terminals. This approach isincapable of producing high aspect ratio protruding contacts because oflimitations of the extrusion method. In addition, dissolution of gold insolder, as taught by this approach creates a problem due to reliabilityconcerns. The method also limits selection of contact material to easilyextrudable metals, like gold.

There are several methods in the prior art for controlled elongation ofsolder masses between a component and a substrate. The goal is to createa column-like solder shape, preferably an hourglass shape, in order toachieve increased resistance to thermal cycling. To that end, U.S. Pat.No. 5,148,968 discloses a web-like device which upon heating duringsolder reflow stretches the solder connections, forming such hourglassshaped solder columns. Aspect ratios of the columns are determined bythe mass of solder in the joint, dimensions of the solder wettableterminals on the substrate and the component, and by the characteristicsof the stretch device. This method is only limited to contact materialswhich are reflowed during the assembly, and requires external hardwarefor forming the contact shape, which adds cost of the hardware andincreases the process complexity.

U.S. Pat. No. 4,878,611 teaches formation of controlled geometry solderjoints by applying controlled volumes of solder to a semiconductorpackage and a substrate, bringing solder masses in contact, andreflowing the solder masses while maintaining controlled separationbetween the component and the substrate through mechanical means. Thisapproach also requires hardware means for maintaining longitudinaldimension of the contacts, and does not lend itself to standardpractices of surface mount soldering, in which industry already has madesubstantial time and capital investments.

In the same spirit, U.S. Pat. No. 4,545,610 discloses a process forforming elongated solder joints between a plurality of solder wettableterminals on a semiconductor component and a substrate by means ofsolder extenders, which transform the shape of the solder joints intouniform hourglass shapes during a solder reflow step. This approachrequires additional, non-standard processing of either a silicon device,or on a substrate, which includes attachment of reflowable solderextenders, and non-reflowable means of maintaining vertical spacingbetween the silicon device and the substrate.

U.S. Pat. No. 5,154,341 discloses the use of a spacer bump which iscomposed of a solder alloy which does not melt at the temperature ofcomponent soldering. A eutectic solder microball is placed on a contact,and upon the reflow the reflowable solder encases the non-reflowablespacer, and produces an hourglass shaped joint. The aspect ratios of thejoint contacts are controlled by dimensions of the spacer.

U.S. Pat. No. 4,332,341 teaches solid state-bonding of solder preformsto components, substrates or both, for further joining. Resultingprotruding solder contacts either collapse during soldering if theyconsist of eutectic solder, or do not reflow at all, when they consistof higher melting temperature solder. In the latter case, a componentwould not have the benefit of a self-alignment effect as the pool ofsolder is confined to the vicinity of the terminals, and a main portionof the joint which controls aspect ratio, does not melt.

U.S. Pat. No. 4,914,814 discloses a process for fabricating soldercolumns through use of a mold with an array of pin holes, which arefilled with solder, brought into contact with terminals on a component,and bonded by melting the solder which wets the terminals. The componentwith columns is then bonded to a substrate through reflow in a solderwith lower melting temperature than the solder of the columns. Thisapproach requires generating a mold for each solder column arraypattern, commonly involving undesirable non-recurring engineeringexpenses and associated increase in delivery times.

U.S. Pat. No. 3,509,270 discloses a printed circuit assembly which isinterconnected with a dielectric carrier that has a plurality of springelements positioned in its apertures, the springs are optionally encasedin a solder material to facilitate permanent electrical contact betweencircuit elements. This approach requires a custom interposer patternmanufactured for each application, while solder coated springs wouldhave to be placed individually inside the apertures. Additionally,soldering usually requires flux, while the interposer material makes itdifficult to clean flux after the reflow process.

U.S. Pat. Nos. 4,664,309 and 4,705,205 disclose a device and a methodfor interconnection with reinforced solder preforms which use aretaining member provided with apertures. The retaining member isoptionally dissolved to leave resilient interconnect structures. Thesolder columns maintain their shape in the molten state, supported byparticles, filaments, spiral metallic wire or tape. 20 to 80% by weightof filler material is specified. This approach, as several of the abovedescribed approaches, requires a custom made retaining member for everyinterconnect application, and therefore requires an additionalnon-recurring engineering expense and increased delivery time for everyproduction order.

U.S. Pat. No. 4,642,889 teaches use of an interposer with a plurality ofinterconnect areas, interconnect areas comprise wire means surroundedwith solder, and incorporate a soldering flux material. Upon heating thesolder reflows and connects to the terminals of the mating componentsand boards, while the wires in the middle of each solder joint insure acolumn-like joint shape. The interposer preferably is dissolved away.While providing means for controlled aspect ratio interconnect joints,this approach also requires use of a custom manufactured interposer andinevitable non-recurring engineering expenses and increased deliverytimes. In addition, when improved alignment of the interconnect to theterminals is required, as terminal-to-terminal distances decrease, thisapproach suffers from heat and environmental effects on interposermaterial, causing it to distort or change dimension during processingsteps, which makes the alignment difficult, and limits this approach torelatively coarse terminal-to-terminal pitch applications.

An elongated protruding solder contact is taught according to U.S. Pat.No. 5,130,779, by sequentially encapsulating solder deposits with abarrier metal. This approach insures that a staged solder deposit doesnot collapse upon reflow, and a controlled aspect ratio solder contactis thus formed. In order for the barrier metal to be effective, thewalls of sequentially deposited solder masses must have a slope.Deposition of such a structure of solder is a lengthy and expensiveprocess.

A method of attaching highly compliant leads to an array matrix ofcontacts on a semiconductor package is disclosed in U.S. Pat. No.4,751,199. An array of leads is manufactured according to the requiredpattern with temporary tab structures tying the leads together.Compliant leads are gang bonded to terminals on the semiconductorpackage, and tabs are removed leaving compliant protruding contactleads, ready for attachment to a substrate. This approach, like theinterposer techniques described above, requires specific tooling forevery new package with a distinct pattern of the array of contacts.

OBJECTS AND SUMMARY OF THE INVENTION

One object of the present invention is to provide a high volume, lowcost manufacturing process for production of precise shape and geometryprotuberant conductive contacts on a wide variety of electroniccomponents, the contacts having a controlled set of physical,metallurgical and mechanical properties, bulk and surface. Suchprotuberant contacts can be employed to satisfy electrical, thermal orgeometrical requirements in various aspects of electronicinterconnection applications. An inventive, multiple stage process isprovided, according to which, first, wire stems are bonded to contactcarrying terminals, the stems being shaped in three dimensional space,to define skeletons of the resulting contacts. The conductive contactsare then completed through at least one deposition step of a coatingwhich envelopes or jackets the wire skeletons and the terminals. Thecommon coating not only helps to “anchor” protruding contacts to theterminals, but also provides for characteristics of the protuberantcontacts with respect to long term stability of their engagement contactwith mating electronic component, including but not limited tointerconnection substrates, semiconductor devices, interconnect or testsockets. The coating also determines soldering assembly characteristics,as well as long term effects from contact with solder. The coating,along with the wire material properties, determines the mechanicalcharacteristics of the resulting protuberant, contact. The wire servesas a “skeleton” of a resulting protruding contact, and the coatingserves as its. “muscle”.

The wire skeleton can be bonded employing high productivity, highlyautomated ultrasonic, thermosonic and thermocompression wirebondingequipment (hereinafter referred to as ultrasonic wirebonding equipment).The wirebonding equipment can be organized for ball bonding typeultrasonic wirebonding process, typically used for gold or copperwirebonding, but modifiable for use with other types of single phase ornoble metal coated wires. Wire skeletons consisting of free standingsingle or multiple wire stems can be produced. As required by the shapeand dimensions of the resulting conductive contacts, the stems can bestanding normal to a terminal or at an angle thereto, or can be formedin three dimensional space. Alternatively, wire skeletons produced usingan ultrasonic ball bonder can be produced by forming a traditionalwire-bonding loop, which originates and terminates on a contact carryingterminal, or which originates on the contact carrying terminal andterminates on a sacrificial conductor outside the terminal area. Thenthe sacrificial conductor is selectively dissolved away after completionof the contact processing. Optionally, a wedge bonding type ultrasonicwirebonder can be employed, in which case controlled shape loops of wireare formed with both ends of a loop on the same terminal, or they areformed by originating the loops within the area of the terminal andterminating or severing the loops outside the terminals. Both types ofwirebonders are available commercially from a variety of suppliers, andare highly automated, with the bonding speeds exceeding 10 wires persecond.

An additional object of the present invention is to produce arraypatterns of contacts for unique component designs, without incurringcosts and delays associated with production of unique tooling. Thisobject is achieved through use of a sequential wirebonding process forattaching of the wire skeletons to the terminals, and then overcoatingthem in a single, non-pattern-specific step. Location targeting andgeometric characteristics are entered into an electronic control systemof the wirebonding equipment with a specific set of commands. A programcontrols all aspects of wire forming and shaping process, as well asbond characteristics. Production of the multiple protruding contactsdoes not require time-limiting and costly production of molds, masks orthe like, which otherwise would be specific to every incoming order.This object is exemplified by application of the present invention toproduction of integrated pin-shaped contacts for ceramic pin grid arraypackages. This is achieved by bonding straight vertical, single stemskeletons, perpendicular to the surface of terminals, and overcoatingthem with a conductive structural metal or alloy, consistingsubstantially of nickel, copper, cobalt, iron or alloys thereof, withthe thickness of the deposit determined by the required pin diametervalue. This can also be accomplished with equal ease on plastic pincarrying substrates.

Yet another object of the present invention is to create protruding,tower-like solder contacts which substantially maintain a wellcontrolled aspect ratio even when the solder is molten. This object isachieved through first bonding a wire skeleton which will define thefinal shape and aspect ratio of a solder contact, then optionallyovercoating it with a barrier layer which prevents long term reactionbetween the solder and the wire, and finally depositing a mass of solderwhich wets the skeleton with a barrier, and clings onto the skeletoneven after multiple reflows.

Yet another object of the present invention is to create controlledshape resilient contacts on top of (or otherwise protruding from)contact carrying terminals arranged in various patterns, includingarrays. This object is achieved by overcoating the wire skeleton with atleast one coating having a predetermined combination of thickness, yieldstrength and elastic modulus to insure predetermined force-to-deflectioncharacteristics of resulting spring contacts.

Yet another object of the present invention is to produce protrudingcontacts on top of (or otherwise protruding from) a multitude ofcontact-carrying terminals, where the terminals have varying verticalcoordinates, corresponding to an origin point of the protrudingcontacts, while uppermost points of the protruding contacts extend to avertical coordinate which is substantially identical, within apermissible tolerance level for the component or substrate which is tobe subsequently connected to the contacts. In a different embodimentrelated to this object, the terminals can originate on variouselectronic components, while the protuberant contacts extend to asubstantially identical vertical coordinate. This object is achieved bycontrolling a wire severing step, by always severing the wire at apredetermined vertical coordinate. Overcoatings then follow the skeletonwire shape and geometry, yielding a contact having a controlled verticalcoordinate level, regardless of the Z-axis point of origination of thecontact, e.g. plane of the terminal. Some of the terminals havingvarying Z-coordinates and overlying the component, can optionally andadditionally overlie single or multiple electronic devices, placed onthe electronic component. In another embodiment related to this objectof the present invention, the vertical coordinate of the tips of theprotuberant contacts can be controlled by the software algorithm of theappropriately arranged automated wirebonding equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages will appear more fullyfrom a Summary of the Invention which follows viewed in conjunction withdrawings which accompany and form part of this Application. In thedrawings:

FIGS. 1 a and 1 b are schematic representations of a knownball-and-wedge technique to form a loop-like wire skeleton of aprotruding electrical contact.

FIG. 2 is a cross-sectional representation of a conductive contactresulting after overcoating the wire skeleton depicted in FIG. 2.

FIG. 3 is a schematic representation of a conductive contacts withskeletons consisting of two loop-like stems.

FIG. 4 is a cross-sectional representation of protruding solder contactsbased on loop-like skeletons with a barrier metal between the wirematerial and the solder.

FIG. 5 is a schematic representation of protruding solder contacts whereeach skeleton consists of two loop-like stems, the skeleton materialovercoated with a barrier layer prior to deposition of solder.

FIGS. 6 a, 6 b and 6 c depict a sequence of bonding one end of aloop-like stem to a sacrificial pad, overcoating the skeleton, and afollowing step of removal of a sacrificial pad, resulting in protuberantcontact with a floating end.

FIGS. 7 a, 7 b and 7 c represent schematically a sequence of bondingsingle stem, perpendicular skeletons to contact-carrying terminals.

FIG. 8 is a cross-sectional view of vertical protuberant contactsproduced by overcoating the skeletons depicted in FIG. 7 c.

FIGS. 9 and 10 schematically represent an example of the skeletons basedon two single stems perpendicular to the plane of contact carryingterminals, and the protuberant contacts resulting after overcoatingskeletons shown in FIG. 9 with contact material.

FIG. 11 is a schematic representation of protuberant contactsoriginating on terminals lying in different planes, and with tips of thecontacts lying in a common horizontal plane.

FIG. 12 is a schematic representation of protuberant contactsoriginating on terminals lying two distinct electronic components, andwith tips of the contacts lying in a common horizontal plane.

FIG. 13 is a schematic representation of a solder protuberant contactbased on a double-stem skeleton, with a barrier layer between theskeleton and the solder.

FIG. 14 is a schematic of S-shaped wire skeletons.

FIG. 15 is a cross-sectional representation of resilient protuberantcontacts resulting after overcoating the S-shaped skeletons with anappropriate conductive material.

FIG. 16 is a cross-sectional representation of resilient protuberantcontacts with microprotrusions for improved contact characteristics.

FIG. 17 is a representation of a fence-like skeleton on top of acontact-carrying terminal.

FIG. 18 is a cross-sectional representation of a pin grid array packageincorporating pins produced according to the method of the presentinvention.

FIG. 19 is a schematic representation of an assembly of a semiconductorpackage incorporating protuberant solder contacts and an interconnectionsubstrate, after completion of the solder reflow step.

FIG. 20 is a schematic representation of an electronic package,incorporating protuberant solder contacts on its two interconnectionsurfaces.

FIG. 21 is a schematic representation of a dielectric interposer withresilient protuberant contacts on its two surfaces, the interposerincluding means of electrical interconnection between the two surfaces.

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention relies on wirebonding equipment toproduce controlled aspect ratio and shape wire skeletons, which aresubsequently overcoated with a desired material in order to produce arequired set of properties for protuberant electrical contacts. FIGS. 1Aand 1B depict use of a ball-and-wedge wirebonding machine to form a wireskeleton. More detailed descriptions of this and wedge-wedge wirebondingmethods, commonly used in the semiconductor industry for interconnectingsilicon devices to packages, can be found in Electronic Packaging andInterconnection Handbook edited by Charles A. Harper, on pp. 6.62–6.64and 7.28–7.30. FIG. 1A illustrates a capillary 1, with an open clamp 2,containing an end portion of a continuous wire 20 with a ball 21 formedat its feed end below the capillary tip. The ball 21 is brought incontact with a contact carrying terminal 90 on top of (or otherwisecontained within) a substrate 10. As a result of application ofpressure, temperature or ultrasonic energy, or combinations thereof, aball is bonded to the terminal. The ball 21 in FIG. 1A is in the bondingprocess changed into ball bond 22 shown in FIG. 1B. Subsequent capillarymotion sets the shape of a wire stem 60. Loop geometry of the wire stemis controlled by parameter settings in the software control algorithm ofthe wirebonding equipment. A second end of the wire stem is bonded tothe terminal 90 by means of pressure, temperature or ultrasonic energy,and deforming the wire against the terminal 90. A wedge-shaped joint 23thereby is produced. The capillary then rises to a predetermined height,a clamp 2 closes, and the wire is severed at a thinnest point of thejoint 23, leaving a fractured free end 24 below the capillary 1. Inpreparation for forming a next joint, a next ball 21 is formed at thesevered free end 24 of the of the wire underneath the capillary, and thecycle is repeated. The loop-shaped stems can be alternatively producedby wedge-wedge technique, where both end of the stem are bonded by thewedge tool. Wire material for ball-wedge and wedge-wedge type ofwire-bonding is most commonly gold, aluminum or copper, with slightmodifications by other elements, like beryllium, cadmium, silicon andmagnesium to control the properties other materials including solder,and specifically lead-tin solder wire, have been employed. Alloys ofsilver and platinum group elements can also be used for wire material.Gold, aluminum and copper, or alloys based on these metals, are thepreferred wire materials. The terminal material should preferably use atleast a top layer (if a multi-layer structure) of gold or aluminum, butnumerous other metallizations can be successfully used, requiringdifferent levels of ultrasonic vibration, force and temperature. Amodern automated wirebonder is capable of bonding over 10 loop-shapedstems per second.

In the embodiment of the present invention represented in FIGS. 1A and1B, a wire skeleton 30 is formed upon severing the single wire stem 60.As shown in FIG. 2, the skeleton 30 geometrically defines theprotuberant contact produced by the method of the present invention.

Referring now to FIG. 2, physical, finishing and determining mechanicaland chemical properties of the resulting protuberant contact 50 areprovided through overcoating of the skeleton 30 and the contact carryingterminal 90 with a continuous coating 40, which consists of at least oneelectrically conducting layer. The continuous coating 40 anchors theskeleton to the terminal by bridging in the areas of contact between theball bond 22 and wedge bond 23 and the terminal. The overcoatingmaterial may be significantly stronger than the skeleton material. Itcan be applied by wet electrochemical means, e.g. through electrolyticor electroless aqueous solution plating of metals on the skeleton andthe terminal. The wet plating techniques are, for instance, described inMetal Finishing Guidebook annually published by Metals and PlasticsPublications, Inc. One preferred embodiment comprises electroplating ofnickel out of nickel and its alloys. This method is capable ofdepositing controlled thickness coating with a tensile strength inexcess of 80,000 pounds per square inch. Additional improvement ofmechanical strength of the resulting contact is achieved when a coatingwith a compressive internal stress is deposited, which effectivelyincreases the stress level, required to deform or break a resultingprotuberant electrical contact.

The coating method also optionally includes so-called physical andchemical vapor methods of conductor material deposition. These optionaltechniques are detailed in a book by M. Ohring, The Materials Science ofThin Films, Academic Press, 1992. Coating methods also include andcontemplate deposition of conductors through various decompositionprocesses of gaseous, liquid or solid precursors.

Nickel has a strong tendency to form an oxide and is, therefore, not thebest choice as a contact metal. It requires large contact forces tobreak through the oxide. For low contact force applications, it requiresa second noble or semi-noble coating layer on top. Gold, silver,elements of the platinum group and their alloys are preferred choices asa noble or semi-noble overcoating layer. Similarly, in other instances,multiple layers, comprising conductive overcoating 40, can be selectedto tailor the set of properties of the protuberant contact to a givenapplication.

Within the method of the present invention, a plurality of wire stemscan be employed to create a wire skeleton. Referring now to FIG. 3, twowire stems 60 comprise each skeleton 31. Conductive overcoating 40completes protuberant contact 51.

In a growing number of applications there is a requirement forcontrolled aspect ratio columns of solder on top of an area array ofterminals on ceramic and plastic semiconductor packages. Most often, anarea array of balls made of a solder alloy, commonly a eutectic alloy oflead and tin, is used in surface mounting of components with an array ofterminals to a matching array of contacts on a circuit board. Long termresistance of such solder contacts is determined by the height, shapeand the material characteristics of the solder joints. The method of thepresent invention provides for controlled shape solder contacts formedaround a wire skeleton. Referring now to FIG. 4, wire skeletons 30 arefirst coated with an optional barrier layer 41, and in the followingstep are overcoated by a solder mass 42. The barrier layer 41 inhibitsan undesired reaction between the wire material and the solder mass.This barrier layer is especially important in the embodiment of theinvention providing for gold wire skeletons and a solder mass comprisingan alloy of lead and tin. Due to reactivity between gold and tin, anddetrimental effects of intermetallic, gold-tin compounds on the serviceperformance of solder joints, the reaction between gold and tin must beprevented. A 100 to 1,6000 microinch barrier of a nickel alloy istypically a sufficient deterrent to the reaction between solder and goldfor most applications.

Deposition of solder overcoating can be accomplished, for example, bysending a package or a substrate through a solder wave process cycle ina solder wave equipment. The solder wets the barrier layer 41, andbridges among adjacent wire portions forming the loop-shaped stems, toassume a shape depicted in FIG. 4. The shape does not substantiallychange during subsequent reflow cycles, as long as there is nosignificant reaction between the solder and the barrier coating. Theoverall shape of the solder column depends on area of the terminal andgeometry of the skeleton. Bridging by molten solder masses between theascending and descending branches of loop-shaped stems, and the surfaceof the terminal, enables the skeletons to contain and supportdisproportionately large volumes of solder. A preferred solderprotuberant contact produced by the method of the present invention willcontain more than 70 volume percent of solder, and more preferably over80 volume percent of solder by volume, with the remainder comprising theskeleton and the barrier material.

A preferred embodiment for a solder column contact, depicted in FIG. 4,is a gold based alloy wire loop skeleton 30, with a nickel based coating41, and a near eutectic lead-tin solder mass 42. The gold wire diameterusually ranges between 0.0005 and 0.005 inches, and preferably between0.0007 and 0.003 inches. The nickel overcoat 40 usually ranges inthickness between 0.00005 and 0.007 inches, and preferably between0.000100 and 0.003 inches. Another preferred embodiment is a copperalloy wire skeleton, which can be directly overcoated with solder mass.In another preferred embodiment, copper based wire skeleton is firstovercoated with a nickel based barrier layer, followed by deposition ofthe solder material.

Both types of protuberant contacts, solder contacts 52 shown in FIG. 4,and contacts 50 shown in FIG. 2, can be soldered to an interconnectionsubstrate or a component using surface mount soldering technology.Solder contacts 52 have an advantage in handling prior to processing, inthat solid solder mass 42, FIG. 4, bridging between the wire branches ofthe skeleton and surface of the terminal, makes the contact moreresistant to mechanical damage in handling, e.g. bending and breakage.

Further increase of the overall proportion of solder material inprotuberant solder contacts can be achieved by using multiple wire stemskeletons. Referring now to FIG. 5, two severed stems 60 comprise eachskeleton 31 bonded to a contact carrying terminal 90. An optionalbarrier material. 41 is first deposited on top of the terminal 90 andthe skeleton 31, in order to inhibit a possible reaction between thesolder and the skeleton material. A solder material 42 completes theconstruction of a protuberant solder contact. By presenting moreascending and descending wire branches for solder bridging, the relativevolume content of solder material in the protuberant contact can besignificantly increased.

Protuberant solder contacts 52 and 53 in FIG. 4 and FIG. 5,substantially retain their shape even during subsequent solder reflows,due to strong wetting between the solder and the barrier layer 41. Thisshape retention is despite solder constituting preferably more than 70volume percent of the protuberant contacts 52 and 53, and mostpreferably more than 80 volume percent. This property of protuberantsolder contacts produced by the method or the present invention allowsfor contacts to be put on electronic components prior to device assemblysteps, even if the assembly process involves heating the component 10above the melting temperature of solder material 42.

In the spirit of the present invention, but in yet an alternateembodiment, either the ball or a wedge bond of the wire stem can beformed outside the area of a contact carrying terminal. As shown in FIG.6A, a wedge bond 29 completes a wire stem 61, and upon the severing stepat the bond 29, resulting in formation of a wire skeleton 32. The wedgebond 29 is formed outside the area of the terminal 90 at a terminal 91.The terminal 91 is positioned on top of a sacrificial layer 80, or couldoptionally be sacrificial by being comprised of a dissimilar metal fromthat of terminal 90. In both cases, the sacrificial arrangement istemporarily used for electrical contact to an electrode, in that it isrequired for electroplating of a conductive coating 43, enveloping theskeletons 32 and the terminals 90 and 91, to complete protuberantconductive contacts 54, as illustrated in FIG. 6B. The sacrificial layeris also used for assuring a different Z-axis coordinate for the bonds 22and 29. As illustrated in FIG. 6C after a coating deposition and aremoval step for the sacrificial layer, one of the ends of the wire loopbased contact ends up being suspended above the substrate 10. Thissuspension is especially important for formation of controlled geometryspring contacts, capable of resiliently engaging with mating terminalson a component or a substrate for testing, burn-in or demountableelectrical interconnect in service. The variation of the Z-coordinate ofan end of the wire-loop shaped contact allows for resilient Z-axismovement of a tip of the resulting spring when a force is appliedthereto.

Referring now to FIGS. 7A through 8, another preferred embodiment of themethod for manufacturing controlled aspect ratio protuberant contacts.In this embodiment, only ball bonding is used, the wires are bondedsubstantially vertically. The software of the control system of thewirebonder is programmed to exclude the common wedge bonding step forsevering the wires. Instead the same electronic or hydrogen flame-offused for ball formation prior to the ball bonding is employed to severethe wires at a predetermined height. FIG. 7A depicts a ball bondingcapillary 1, wire 20 after the first ball bonding step which created aball bond 22 on top of contact carrying terminal 11. After the bond thecapillary moves up to a predetermined position, and electrode 3 isbrought under a high potential, resulting in a generation of a sparkwhich melts and severe the wire 20 at a predetermined spot, asillustrated in FIG. 7B. As a result of the severing step a wire skeleton33 is created, which comprises a single stem 62 extending to its severedend 25. The severing step also readies the feed end 21 of the wire 20 tothe next stem bonding step.

After vertical wire stems 62 are formed on contact carrying terminals90, and after the severing step which defines single stem skeletons 33,as shown in FIG. 7C, the wire skeletons 33 and the contact carryingterminals 90 are overcoated with a deposit 44, originating at theterminals and extending to the tops of the wires as a continuous blanketcoating, to complete protuberant contacts 55. As in the previousembodiments, the composition and the thickness of the continuous coatingis selected to satisfy requirements of a given application.

The protuberant vertical contacts 55 shown in FIG. 8 are especiallyuseful as a replacement method for standard techniques for attachment ofpins to plastic and ceramic semiconductor packages, a method whichresults in lower package cost and reduced pinned package productiontime. This usefulness is due to the fact that the pin-shaped contactsproduced by the method of the present invention do not require patternspecific tooling or molds.

Thickness and material composition of conductive layer 44 shown in FIG.8 depends on the production, assembly and service requirement, and thecharacteristics of package material. The layer 44 can comprise nickelalloy. In another embodiment layer 44 can comprise a copper alloy. Yetin another embodiment layer 44 can comprise alloys of nickel, iron andcobalt, with controlled thermal expansion characteristics. In yetadditional embodiment, layer 44 is forme through multiple depositionsteps, the top deposit comprising a noble or semi-noble metal or alloy,out of a group or gold, platinum, silver, rhodium, ruthenium and copper.The top deposit improves electrical contact characteristics of thepin-like contact 55.

FIGS. 9 and 10 illustrate the process of attachment of contacts 56 tothe contact carrying terminals 90. The contacts are based on skeletons34, each skeleton 34 comprising two vertical wire stems 62. Theskeletons and the terminals 90 are overcoated by layer 44 to completeprotuberant, vertical-double-stem contacts 56. This type of contacts canbe useful in soldering, where multiple branches of the contact presentthemselves to solder for improved assembly yield.

Referring again to FIG. 7 b, due to the stationary position of theelectrode 3, the wire 20 always gets severed at a predeterminedelevation, regardless the Z-coordinate of the ball bond 22. Uniformelevation is a very desirable property, especially in cases whencontacts are placed on a substrate or a component which is not planardue to manufacturing tolerances, or is warped during thermal assembly orprocessing steps. In other instances, various terminals 90 on anelectronic component 10 are provided in different planes, as illustratedin FIG. 11, while the highest elevation points of the protuberantcontacts 56 may be required to lie in a substantially identicalhorizontal plane. This configuration is especially important whencontact carrying terminals lie at different positions of aninterconnection substrate, but they bear protuberant contacts which mustcontact a highly planar device or component. This self-planarizingcapability of the method of the present invention is also important whenthe terminals lie on different components, but the tips of protuberantcontacts must terminate in a substantially identical horizontal plane.As illustrated in FIG. 12, terminals 90 and 92 lie on components 10 and11 respectively, while the protuberant contacts 56 vertically terminatein a substantially identical plane. In the embodiment depicted in FIG.12, the two components, 10 and 11, are optionally interconnectedelectrically by means of conductive masses 900. In this embodiment ofthe invention, component 11 can represent an interconnection substrate,while one or a plurality of components 11 can represent passive devices,like capacitors or resistors. The protuberant contacts may serve forinterconnection of a bare, unpackaged semiconductor device (not shown inthe FIG. 12) to the interconnection substrate 10. Such an electricalarrangement decreases the inductance value between critical contacts onthe semiconductor device and a capacitor component, which improveselectrical performance of semiconductor devices operating at highfrequencies.

Wire skeletons consisting of multiple vertical wire stems are especiallyuseful for protuberant solder contact applications. FIG. 13. illustratesa cross section of a protuberant solder contact 57 supported by a wireskeleton 34 consisting of two vertical stems 62. An optional barrierlayer 45 is first deposited, followed by the solder 42 deposition step.The deposition step can be accomplished by passing a substrate withprotuberant skeletons 34, overcoated with a barrier metal 45, through acommon wave soldering machine. The solder bridges between the wires, inaddition to coating the outsides and top surfaces of the wires due towetting. In contrast, a single wire skeleton would support solder onlythrough the wetting mechanism, without a possibility of bridging, andwould support less bulk solder per wire as a result of solderapplication by solder wave technique. A common solder state-of-the-artwave soldering method is described in Electronic Materials Handbook,Volume 1 Packaging, from ASM International, Materials Park, Ohio, on pp.688 through 696. A preferred method for producing this embodiment of thepresent invention involves use of multiple gold wires, ranging from0.0005 to 0.005 inches in diameter, and more preferably 0.0007 to 0.003inches in diameter, overcoated with 0.000030 to 0.005 inches of nickelor nickel alloy or cobalt or cobalt alloy, and more preferably with0.000050 to 0.003 inches of nickel or nickel alloy. The amount of solderdeposited from solder wave would depend on the wave conditions and thedimension of the overcoated skeleton, as well as the size of the contactcarrying terminal.

In another embodiment of the present invention, the solder overcoat 42,which completes a protuberant solder contact 57, is a continuous coatingdeposited over a wire skeleton without a barrier layer between thesolder and the wire and terminals. Gold wire without a barrier is not anappropriate choice for this embodiment, because a continuous reactionbetween the solder and the gold embrittles the solder or solder joint toa substrate or to a component. However, a copper wire is useful to formthe wire skeleton, and then a solder coating is applied using, forexample, a solder wave approach referred to above.

An alternative approach to forming solder columns on top of wireskeletons is to plate electrolytically the solder. The electrolyticallydeposited solder is appropriate for standard surface mount assembly.Alternatively, the solder can be reflowed after electrolytic deposition,and prior to the assembly.

The method of forming wire skeletons described by means of ball bonding,shown in FIGS. 7A through 7C is appropriate for forming individualresilient contacts. Instead of forming vertical wires, the skeletonwires for spring contacts are formed with the shape which deviates fromvertical. One preferred embodiment for forming spring contacts isillustrated in FIGS. 14 and 15. A single or multiple wire is ball bondedto a terminal, followed by a motion of the capillary which forms thewire into an S-shaped wire stem 63. The forming step is followed by asevering step performed by means of an electronic flame-off provided bythe tool 3. The severing step defines a skeleton consisting of single ormultiple S-shaped wire stems. The skeletons 35 and the resilient contactcarrying terminals 90 are then overcoated by a conductive deposit 46,which possesses mechanical characteristics, which, along with thes-shape of the stems, ensures a resilient response of the resultingprotuberant contacts 58 to a deflective force.

One preferred embodiment for a resilient overcoating is a nickel or anickel alloy layer. For example, Ni electroplated out of standard nickelsulfamate solution could be used. Such nickel deposit can be producedwith compressive internal stress which would improve the springcharacteristics, as well as varying strengthand ductility levels. Aplated nickel-cobalt alloy has greater strength and improved resilientproperties. Rhodium, ruthenium or other elements of the platinum groupand their alloys with gold, silver and copper constitute another groupof preferred embodiment overcoat materials. Tungsten or nickel can alsobe deposited by chemical vapor deposition techniques, and representanother preferred embodiment materials.

Gold is the most commonly used wire material for ultrasonic wire bondingapplications, but it is soft and it may not be an appropriate skeletonmaterial for a spring contact if it constitutes a significant portion ofthe spring cross-sectional area. One embodiment of the present inventionprovides for a common high speed bonding of gold skeleton wires. Analloying layer is then deposited, which when reacted with gold, forms agold alloy, the alloy having higher strength then pure gold. Onepreferred embodiment provides for deposition of tin on top of gold wire,with subsequent reaction of gold and tin at a temperature below themelting temperature of gold-tin eutectic. A gold-tin alloy results,which is significantly stronger than gold.

The contact properties of the springs in FIG. 15 can be enhanced byovercoating the spring material with a noble or semi-noble material,like gold, silver, or elements of the platinum group and their alloys.This overcoating reduces contact resistance of spring contacts 58 whenthey are engaged against the mating conductive terminals forinterconnection purposes. Another embodiment is depicted in FIG. 16,with a spring coating 47 having local protrusions. Such a coating can becreated through dendritic growth of an electroplated deposit, or throughincorporation of foreign particulates into the conductive deposit 47.Alternatively, a regular uniform first deposit layer can be applied,which provides for resilient properties, and the subsequently depositedtop layer incorporates local protrusions or particulates to completeconductive deposit 47. The local protrusions dramatically increase thelocal pressures exerted by the resilient protruding contact 59 onto amating terminal during the interconnection engagement, and reducecontact resistance when contacting easily passivating, oxide formingmaterials overlying the engaged terminals.

Resilient protuberant contact produced by the method of the presentinvention rely on the shape of a skeleton and the properties of theconductive material for its spring properties. In another embodiment ofthe present invention, wire stems or wire skeletons can be additionallyshaped by a tool external to a wirebonding equipment, prior or beforethe deposition step.

FIG. 17 illustrates another embodiment of the present invention, where afence-like skeleton 36 is erected on top of a contact carrying terminal90. The skeleton is formed by sequential bonding of the additionalloop-shaped wire stems, without the severing steps between the bonds,until the skeleton is completed. This skeleton shape is especiallyuseful when large masses of solder have to be contained within thespacial boundaries determined by the skeleton 36. One preferredapplication of this embodiment is production of massive solder pads forthermal interconnection to heat sinks or substrates.

Protuberant contacts, as manufactured according to the presentinvention, are mounted on terminals on top of various interconnectionsubstrates; such as laminate printed circuit boards, Teflon basedcircuit boards, multi-layer ceramic substrates, silicon basedsubstrates, varieties of hybrid substrates, and other substrates forintegration of electronic systems known to those skilled in the art. Thecontacts can also be put on top of terminals directly on semiconductordevices, such as silicon and gallium arsenide devices, for subsequentdemountable or permanent attachment to interconnection substrates. Thecontacts could also be put on terminals on one or both sides ofelectronic components or devices, such as ceramic and plastic packageshousing semiconductor components, and other devices. The contacts couldbe put directly on top of passive devices, such as resistors andcapacitors.

One arrangement is shown in FIG. 18, wherein pin-like contacts 55 aremounted to terminals of a ceramic semiconductor package 100. Asemiconductor device 101 is bonded to the package using die attachmaterial 112. Electrical interconnection of the device to the package isaccomplished with industry standard gold wire bonds 113, extending fromterminals 95 on the semiconductor device 101 to terminals 94 on theceramic package 100. This could alternatively be accomplished withaluminum wire wedge bonding. Interconnectivity within the ceramicpackage 100, between the wirebond terminals 94 and pin contact carryingterminals 96, is accomplished through the incorporation of conductors910 within the ceramic package body. The device is sealed hermeticallyin a package cavity with, typically, a Kovar alloy lid 111. The contactscan be put on before or after attachment and assembly of thesemiconductor device 101 in the package 100. The package 100 with theprotruding contacts is ready for interconnection to an interconnectionsubstrate, such as a printed circuit board. Depending on the method ofconnection, top deposit layer of the contacts 55 can be gold, noble orsemi-noble material, tin, or a tin-lead solder alloy.

FIG. 19 illustrates a cross section of a semiconductor package 100, withsemiconductor device 101 interconnected therewith, and interconnected toa substrate 12 with terminals 97, the pattern of the terminals 97matching the pattern of contact carrying terminals 96 on the package100. The package solder column shaped joints between the terminals 96and 97 resulted after solder attachment of the package 100 with columns57 to a substrate 12. The surface mount soldering is accomplishedpreferably by stencil or screen printing solder paste volumes on top ofeach terminal 97, positioning contacts 57 in contact with said solderpaste, reflowing the solder in an oven at a temperature above themelting temperature of the solder in solder paste. Alternatively soldercan be applied by various means of deposition, and placement and reflowmethods can be utilized. The soldering process follows industry standardprocedure commonly referred to as Surface-Mount Technology, anddescribed in Chapter 9 of Electronic Packaging and InterconnectionHandbook, edited by. Charles A. Harper, and published by McGraw-Hill,Inc. As shown in FIG. 19, an hour-glass joint shape results, which iscommonly recognized as the most reliable shape for increased resistanceto thermally induced joint failures.

Due to the fact that columns 57 can be manufactured with any solder,including eutectic tin-lead solder, the most of the volume of thecontact 57 melts during the reflow, and the solder redistributes itselfaccording to the surface area of its reinforcing wire skeleton and thesurface area of its mating terminals. This feature allows one to achievein some cases a self-alignment effect, e.g. the component is pulled intoregistration with the terminal array on a substrate due to surfaceforces of solder wetting. This feature is in contrast to othertechniques for solder column utilization, which typically use highermelting temperature solder, and only the portions of solder depositedonto terminals on a substrate melt, which reduces the absolute value ofsolder-surface wetting forces.

The contacts of the present invention can be put on both sides of anelectronic package or a substrate for multiple interconnection tovarious devices. Alternatively, the contacts can be put on one side ofan electronic package for interconnection to a semiconductor device, andon the other side of the semiconductor package for subsequentinterconnection to a circuit board or any other substrate. FIG. 20depicts an electronic package 110, which has miniature solder columns570 for interconnecting a flip-chip semiconductor device, on top of thepackage, and solder columns 57 at the bottom, for subsequentinterconnection to a printed circuit board.

Spring contacts produced by the present invention are used as a standardmeans of interconnect between substrates and components which havematching patterns of terminals in many cases it is desirable not tomanufacture the contacts on either substrates or components, or devices,as the process yield associated with contact manufacturing would causeloss of costly devices, substrates or components. One embodiment of thepresent invention, illustrated in FIG. 21, provides for a substantiallyplanar interposer 120 with matching set of terminals 97 on both sidesthereof, and means 930 for connecting electrically the matchingterminals on both of the sides. Protuberant resilient contacts 59 are,placed on contact carrying terminals 97 on both sides of the interposer120. This contact carrying structure is ready for demountableinterconnection of a variety of electronic components.

It will be apparent to those skilled in the art that wide deviations maybe made from the foregoing preferred embodiments of the inventionwithout departing from a main theme of invention set forth in claimswhich follow herein.

1. A method of forming a contact structure, said method comprising:building a core structure on a substrate, said core structure defining adesired shape of said contact structure; and over coating at least aportion of said core structure, wherein said over coat comprises amaterial that is one of harder or has a greater yield strength than amaterial of said core structure.
 2. A method claim 1, wherein saidbuilding step comprising: attaching a wire to said substrate; spoolingout said wire; and severing said wire.
 3. The method of claim 2, whereinsaid spooling step comprises spooling out said wire in said desiredshape of said contact structure.
 4. The method of claim 3, wherein saidspooling step further comprises moving a spool from which said wire isspooled in said desired shape of said contact structure.
 5. The methodof claim 4, wherein said moving step comprises moving said spoolhorizontally and vertically with respect to said substrate.
 6. Themethod of claim 3, wherein said desired shape comprises at least onereversal of direction.
 7. The method of claim 2, wherein said attachingstep comprises bonding an end of said wire to said substrate.
 8. Themethod of claim 2, wherein said substrate comprises a terminal and saidattaching step comprises attaching said wire to said terminal.
 9. Themethod of claim 8, wherein: said substrate comprises a plurality ofterminals, and said method further comprises repeating said attaching,spooling, severing, and over coating steps a plurality of times andthereby forming a plurality of contact structures on said plurality ofterminals.
 10. The method of claim 1, wherein said material of said overcoat is harder than said material of said core structure.
 11. The methodof claim 1, wherein said material of said over coat has a greater yieldstrength than said material of said core structure.
 12. The method ofclaim 1, wherein said substrate is a semiconductor device.
 13. Themethod of claim 12, wherein said semiconductor device is a singulateddie.
 14. The method of claim 13, wherein said die is unpackaged.
 15. Themethod of claim 12, wherein said semiconductor device is a die of anunsingulated semiconductor wafer.
 16. The method of claim 1, whereinsaid building step comprises one of forming, creating, or disposing saidcore structure on said substrate.
 17. A method of forming a contactstructure, said method comprising: attaching a wire to a surface of asubstrate; spooling out said wire from a spool; while spooling out saidwire, moving said spool in a motion that comprises both a verticalmovement and a horizontal movement with respect to said surface of saidsubstrate to impart a desired shape of said contact structure to saidwire; and severing said wire.
 18. The method of claim 17, wherein saidshape comprises at least one reversal of direction.
 19. The method ofclaim 17, wherein said attaching step comprises bonding an end of saidwire to said substrate.
 20. The method of claim 17, wherein said surfaceof said substrate comprises a terminal and said attaching step comprisesattaching said wire to said terminal.
 21. The method of claim 19,wherein: said surface of said substrate comprises a plurality ofterminals, and said method further comprises repeating said attaching,spooling, moving, and severing steps a plurality of times and therebyforming a plurality of contact structures on said plurality ofterminals.
 22. The method of claim 19, wherein said substrate is asemiconductor device.
 23. The method of claim 21, wherein saidsemiconductor device is a singulated die.
 24. The method of claim 22,wherein said die is unpackaged.
 25. The method of claim 21, wherein saidsemiconductor device is a die of an unsingulated semiconductor wafer.